Motion estimation method

ABSTRACT

For de-interlacing a current interlaced field using a prior interlaced field and a subsequent interlaced field, a motion estimation method is performed with a group of motion-vector sets. Each motion-vector set includes at least two motion vectors. According to the two motion vectors and a specified block of the current interlaced field, which contains a missing pixel to be estimated, a pair of candidate blocks are defined in the prior and subsequent interlaced fields, respectively. By comparing a plurality of pairs of candidate blocks with the specified block, the best matching block pair can be found and used for de-interlacing.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This patent application claims the benefit of U.S. provisional patentapplication No. 60/909,945 filed Apr. 4, 2007.

FIELD OF THE INVENTION

The present invention relates to a motion estimation method, and moreparticularly to a motion estimation method for use in a de-interlacingprocess.

BACKGROUND OF THE INVENTION

Interlaced and non-interlaced (or progressive) image formats are twogeneral image formats, and need to be processed by a variety of imagesystems when transmitted among those image systems. An interlaced imagecan be converted into a non-interlaced image by a de-interlacingoperation. For example, referring to FIG. 1A, by estimating respectivemissing scan lines (as indicated by dashed lines) of an odd-fieldinterlaced image field 11 and an even-field interlaced image field 12through a de-interlacing operation 13, two non-interlaced image frames14 and 15 with a full number of scan lines can be obtained.

The de-interlacing operation 13 can be implemented by way ofinterpolation. More specifically, a pixel value of a missing pixel canbe estimated by way of interpolation according to pixel values ofadjacent pixels in upper and lower existing lines, which is referred toas a line-averaging method. Such a de-interlacing method, however, islikely to cause image distortion and/or flicker problems. For solvingthese problems, block-based motion estimation conventionally used forvideo compression is applied for de-interlacing. Since two continuouslydisplayed image fields are generally highly correlated to each other,the block-based motion estimation is performed based on adjacent fields.

A scheme of block-based motion estimation is depicted in FIG. 1B. In theblock-based motion estimation method, a block 18 with a preset size, forexample 8*8 pixels, is specified in the current field (n) with a pixelto be estimated centering thereon. On the other hand, a plurality ofcandidate blocks within a search window 17 are located according toinformation of the specified block 18 and a plurality of preset vectors,respectively. For each pair of pixels at corresponding positions of thespecified block 18 in the current field (n) and each of the candidateblocks of the preceding field (n−1), a difference in pixel values suchas RGB values is calculated. Then absolute values of all the pixeldifference values of each candidate block are summed to determine ablock difference value. By comparing the block difference values of thecandidate blocks, the one having the smallest block difference valuefrom the specified block 18 can be determined as a best matching block19. Meanwhile, the motion vector 16 indicating the best matching block19 is an optimal motion vector.

When applying the block-based motion estimation to a de-interlacingprocess, wherein the fields (n−1) and (n) are of opposite parity fields,a problem that there are no simultaneously existing pixels atcorresponding positions of the specified block and the candidate blocksis encountered. Therefore, a variety of methods for calculating blockdifference values have been developed to determine the best matchingblock and motion vector.

Please refer to FIG. 2A, wherein column pixels of two continuousinterlaced fields of opposite parity fields at corresponding positionsare shown side by side in the plot. The same references (y−3)˜(y+3)represent pixels at the same positions of the two interlaced fields (n)and (n−1), wherein pixels (y−3)˜(y+3) are a series of adjacent pixels inthe same column. In the column of the interlaced field (n), solidcircles represent existing pixels while dashed triangles representmissing pixels; and in the column of the preceding interlaced field(n−1), solid circles represent existing pixels while dotted circlesrepresent missing pixels. The missing pixels in the field (n−1)represented by the dotted circles have been de-interlaced previously ina similar motion-estimation way. Based on currently available colorvalues, block-based motion estimation is performed as described abovewith reference to FIG. 1B. For example, for estimating the pixel (n,y)of the interlaced field (n), a specified block containing and centeredon the pixel (n,y) and a plurality of candidate blocks in the precedinginterlaced field (n−1) are defined. Then respective block differencevalues between the specified block and the candidate blocks arecalculated. Among them, the smallest block difference value isdetermined, and the best matching block is thus the candidate blockhaving the smallest difference value. Accordingly, the optimal motionvector, e.g. the vector C_(P), can be obtained.

Since the pixel (n,y) is actually missing in the interlaced field (n),its own pixel difference value cannot be calculated. Instead, pixelvalues of existing pixels in the specified block are used forcalculating the block difference values along with pixel values of thecorresponding pixels in the candidate blocks. The corresponding pixels,however, are actually missing but previously de-interlaced. Therefore,the block-based motion estimation is based on estimated data, which ifnot well de-interlaced, would result in error propagation and even imagedistortion.

FIG. 2B illustrates another method for searching a best matching blockand determining an optimal motion vector. As a matter of fact, a bestmatching pair of blocks, instead of a best matching block, are searchedin this prior art. Column pixels of three continuous interlaced fields(n−1), (n) and (n+1) at corresponding positions are shown side by sidein the plot. In this prior art, interfaced fields (n−1) and (n+1) of thesame field are used for obtaining the estimated motion values and motionvectors of the target interlaced field (n). A plurality of pairs ofcandidate blocks are respectively searched from the interfaced fields(n−1) and (n+1), wherein each pair of candidate blocks have respectivecenter pixels aligned with the pixel (n,y) in the same line. Thenrespective block difference values between the pairs of candidate blocksare calculated. Among them, the smallest block difference value isdetermined, and the best matching pair of blocks is thus the pair ofcandidate blocks having the smallest difference value. Accordingly, theoptimal motion vector, e.g. the vector C_(B), can be obtained. In thisprior art, the missing pixel values of the interlaced field (n) aredetermined according to the pixel values of adjacent fields (n−1) and(n+1) while involving no existing pixel values of the interlaced field(n) itself. Therefore, errors are likely to occur particularly when thepixel to be estimated does not exist in the neighbor fields (n−1) and(n+1).

SUMMARY OF THE INVENTION

Therefore, the present invention provides a motion estimation method foruse in a de-interlacing process with improved precision.

The present invention provides a motion estimation method for use in ade-interlacing process of a current interlaced field. The methodincludes steps of defining a specified block in the current interlacedfield, wherein the specified block contains a missing pixel to beestimated and at least one existing pixel; pre-estimating missing pixelsof a prior interlaced field and a subsequent interlaced field; defininga first candidate block in the prior interlaced field with pre-estimatedpixel values and defining a second candidate block in the subsequentinterlaced field with pre-estimated pixel values, wherein a position ofthe first candidate block in the prior interlaced field and a positionof the second candidate block in the subsequent interlaced field aredetermined according to a position of the specified block in the currentinterlaced field and a motion-vector set; calculating a first blockdifference value between the at least one existing pixel of thespecified block and at least one corresponding pixel of the firstcandidate block with pre-estimated pixel values or at least onecorresponding pixel of the second candidate block with pre-estimatedpixel values; calculating a second block difference value between the atleast one corresponding pixel of the first candidate block withpre-estimated pixel values and the at least one corresponding pixel ofthe second candidate block with pre-estimated pixel values; andcalculating a motion estimation value corresponding to the motion-vectorset according to the first block difference value and the second blockdifference value.

The present invention also provides a motion estimation method for usein a de-interlacing process, which includes: receiving a firstinterlaced field, a second interlaced field and a third interlaced fieldin sequence; calculating and filling estimated pixel values of missingpixels of the first and third interlaced fields; defining a specifiedblock in the second interlaced field which consists of a plurality ofexisting pixels; selecting a motion-vector set from a group ofpredetermined motion-vector sets; defining a first candidate block inthe first interlaced field at a position relative to that of specifiedblock in the second interlaced field according to the selectedmotion-vector set; defining a second candidate block in the thirdinterlaced field at a position relative to that of the specified blockin the second interlaced field according to the selected motion-vectorset; generating a first block difference value by comparing pixel valuesof the first candidate block with pixel values of the specified block;generating a second block difference value by comparing pixel values ofthe second candidate block with pixel values of the specified block;generating a third block difference value by comparing pixel values ofthe first candidate block with the pixel values of second candidateblock; and determining whether the selected motion-vector set is anoptimal motion-vector set according to the first, second and third blockdifference values.

The present invention further provides a motion estimation method foruse in a de-interlacing process, which includes: receiving a firstinterlaced field, a second interlaced field and a third interlaced fieldof a video signal; defining a specified block in the second interlacedfield; defining a plurality of pairs of first and second candidateblocks in the first and third interlaced fields, respectively, accordingto the specified block and a plurality of motion vector sets;calculating a plurality of first block difference values between thespecified block and the first candidate blocks; calculating a pluralityof second block difference values between the specified block and thesecond candidate blocks; calculating a plurality of third blockdifference values between the pairs of first and second candidateblocks; determining a provisionally optimal motion vector according tothe first, second and third block difference values; comparing one ofthe first block difference values, one of the second block differencevalues and one of the third block difference values, which correspond tothe provisionally optimal motion vector, with a first threshold and asecond threshold lower than the first threshold; and determining whetherthe provisionally optimal motion vector set includes an optimal motionvector or not according to the comparing result.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more readily apparent to thoseordinarily skilled in the art after reviewing the following detaileddescription and accompanying drawings, in which:

FIG. 1A is scheme showing an de-interlacing operation;

FIG. 1B is a scheme showing a typical block-based motion estimationmethod;

FIG. 2A is a plot schematically showing associated pixels used in aconventional block-based motion estimation method;

FIG. 2B is a plot schematically showing associated pixels used inanother conventional block-based motion estimation method;

FIGS. 3A˜3C are schematic diagrams showing three consecutive interlacedfields involved in a motion estimation method according to an embodimentof the present invention;

FIGS. 4A˜4C are schematic diagrams showing distributions of existing andmissing pixels in three associated blocks involved in a motionestimation method according to an embodiment of the present invention;

FIG. 5 is a flowchart briefly summarizing a motion estimation methodaccording to an embodiment of the present invention;

FIG. 6 is a scheme illustrating a case in which error may happen due toa totally or partially hidden object; and

FIG. 7 is a flowchart summarizing a motion estimation method accordingto another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For improving accuracy of motion estimation, the present inventionprovides a multi-directional motion estimation method. In an embodimentof the present invention, a set of three motion vectors are used fordefining a candidate block. Please refer to FIGS. 3A, 3B and 3C, inwhich three consecutive interlaced fields 31, 32 and 33 are shown. Thefields 31 and 33 are of the same parity field, while the field is of anopposite parity field. In other words, if the fields 31 and 33 are of aneven field, the field 32 is of an odd field; or the fields 31 and 33 areof an odd field, and the field 32 is of an even field.

First of all, a block 320 with a preset size is specified in the currentfield 32 of FIG. 3B. The block 320 consists of a plurality of existingpixels and a plurality of missing pixels, which are indicated by circlesand triangles in FIG. 4B, respectively. For example, when the field 32is an interlaced field with an 800*600 pixel size, it actually includes800*300 pixels. In this case, it is proper that the specified block 320has a block size of 4*4, 8*8 or 16*16 pixels, wherein only 4*2, 8*4 or16*8 pixels are actually existent and the others are missing pixels tobe estimated.

On the other hand, the preceding and next interlaced fields 31 and 33also have substantially a half of pixels missing, so interpolation isperformed to estimate pixel values of the missing pixels. Theinterpolation in this embodiment is additionally performed by way ofline averaging, i.e. estimating a pixel value of a missing pixelaccording to pixel values of adjacent pixels in upper and lower existinglines.

Furthermore, a plurality of sets of three motion vectors (C_(P), C_(B)and C_(N)) are provided, wherein C_(P) and C_(N) are in oppositedirections and starts from an existing pixel of the field 32, whileC_(B) is parallel to C_(P) and C_(N) and starts from an existing pixelof the field 31 or an existing pixel of the field 33. Based on arelative position of the block 320 in the field 32, for examplerepresented by a left upper corner pixel and indicated by a coordinate3200, and one of the sets of motion vectors, a candidate block 310 isdefined in the field 31, as shown in FIG. 3A, and a candidate block 330is defined in the field 33, as shown in FIG. 3C. The motion vector C_(P)points from the coordinate 3200 in the field 32 to a coordinate 3100 inthe preceding field 31, thereby defining the candidate block 310.Likewise, the motion vector C_(N) points from the coordinate 3200 in thefield 32 to a coordinate 3300 in the preceding field 33, therebydefining the candidate block 330.

In response to the vector C_(P), a first block difference value iscalculated by summing absolute values of the pixel difference valuesbetween existing pixels of the field 32 and corresponding pixels of thefield 31. The corresponding pixels of the field 31 could be existingpixels or missing pixels with estimated pixel values, depending on thevector C_(P). Likewise, in response to the vector C_(N), a second blockdifference value is calculated by summing absolute values of the pixeldifference values between existing pixels of the field 32 andcorresponding pixels of the field 33. The corresponding pixels of thefield 33 could be existing pixels or missing pixels with estimated pixelvalues, depending on the vector C_(N). Furthermore, in response to thevector C_(B), a third block difference value is calculated by summingabsolute values of the pixel difference values between existing pixelsof the field 31 and corresponding existing pixels of the field 33. Thefirst, second and third block difference values are then summed toobtain a motion estimation value corresponding to the set of threemotion vectors. In a similar manner, other motion estimation valuesrespectively corresponding to other sets of motion vectors arecalculated and compared to one another. Among them, the smallest motionestimation value is determined, and the best matching pair of blocks isthus the candidate block pair resulting in the smallest differencevalue, e.g. blocks 310 and 330. Accordingly, the optimal set of motionvectors C_(P), C_(B) and C_(N) can be obtained. A flowchart of FIG. 5summarizes the principal concept of the above motion estimation method.

The term “corresponding pixels” used above indicates the pixels at thesame relative positions in different blocks of the same size. Any othertype of correspondence can also be applied as long as the correspondingpixels are highly correlated in desired properties such as color values.In the above embodiment, all existing pixels in the specified block 320and corresponding pixels in the candidate blocks are used forcalculating the block difference values. Alternatively, it is alsofeasible to use one or more selected pixels only to calculate the blockdifference values. In addition, a set of two motion vectors can be usedin lieu of the set of three motion vectors in order to simplify thecalculation. The number of sets of motion vectors is also adjustabledepending on practical requirements.

FIGS. 4A, 4B and 4C exemplify the specified block 320 and candidateblocks 310 and 330, respectively. In this example, each of the blockshas an 8*8 pixel size. The circles in each of the blocks indicateexisting pixels while triangles and rectangles indicate missing pixelsto be estimated. In other words, as shown, the odd lines of thecandidate blocks 310 and 330 and the even lines of the specified block320 are missing pixels. Alternatively, varying with the motion vectors,i.e. change of coordinate 3100 and/or 3300, it is possible that the oddlines of the candidate blocks 310 and 330 are existing pixels while theeven lines are missing pixels. Basically, the blocks 310 and 330 changesimultaneously. On the other hand, the even lines of the specified block320 can be existing pixels while the odd lines can be missing pixelsonce the specified block (or coordinate 3200) changes.

Although an interlaced field can be precisely de-interlaced by findingan optimal motion vector set as mentioned above, the de-interlacingoperation can be further improved in some special cases. For example, inan embodiment of the present invention, three consecutive interlacedfields of a video signal are involved for estimating pixel values ofmissing pixels of a current interlaced field. However, if the videosignal includes a motion object that appears in the first interlacedfield and the second interlaced field but disappears from the thirdinterlaced field, or the motion object is not present in the firstinterlaced field but appears in the second and third interlaced fields,the motion estimation involving all the three interlaced fields wouldbecome impractical enough.

FIG. 6 illustrates another case in which errors may happen. As shown, amotion object 60 moves relative to a background 61, and thus exists atdifferent positions of the first interlaced field and the secondinterlaced field but is partially hidden from a static object 62 in thethird interlaced field. Under this circumstance, the optimal motionvector set determined according to the above-mentioned motion estimationmethod is used as a provisionally optimal motion vector set whichrequires modification. Since the motion object 60 is only partiallyshown in the third interlaced field, the first block difference valuecalculated in response to the vector C_(P) by summing absolute values ofthe pixel difference values between existing pixels of the secondinterlaced field and corresponding pixels of the first interlaced fieldis much smaller than the second block difference value calculated inresponse to the vector C_(N) by summing absolute values of the pixeldifference values between existing pixels of the second interlaced fieldand corresponding pixels of the third interlaced field and the thirdblock difference value calculated in response to the vector C_(B) bysumming absolute values of the pixel difference values between existingpixels of the first interlaced field and corresponding existing pixelsof the third interlaced field. In a further embodiment of the presentinvention, for alleviating the adverse effect of the second and thirdblock difference values, the first block difference value is solely usedas the motion estimation value, and an optimal motion vector C_(P) canbe determined by acquiring and comparing a plurality of motionestimation values corresponding to a variety of preset motion vectorsC_(P). The motion vector resulting in the smallest motion estimationvalue is determined as the optimal motion vector C_(P).

In contrast, if the motion object 60 does not exist in the firstinterlaced field but exists at different positions of the secondinterlaced field and the third interlaced field, the second blockdifference value will be much smaller than the first block differencevalue and the third block difference value. Therefore, it is preferredthat the second block difference value is solely used as the motionestimation value, and an optimal motion vector C_(B) can be determinedby acquiring and comparing a plurality of motion estimation valuescorresponding to a variety of preset motion vector C_(B). The motionvector resulting in the smallest motion estimation value is determinedas the optimal motion vector C_(B).

A flowchart of FIG. 7 summarizes the modified motion estimation method.First of all, the first, second and third block difference valuescorresponding to the provisionally optimal motion vector set arecompared with a first threshold and a second threshold that is lowerthan the first threshold (Step S71). If the second and third blockdifference values are greater than the first threshold and the firstblock difference value is smaller than the second threshold (Step S72),an optimal motion vector is determined solely according to the firstblock difference value (Step S75). Otherwise, check if the first andthird block difference values are greater than the first threshold andthe second block difference value is smaller than the second threshold(Step S73). If positive, determine an optimal motion vector solelyaccording to the second block difference value (Step S74). Otherwise,confirm the provisionally optimal motion vector set to be the optimalmotion vector set, and obtain an optimal motion vector for subsequentoperations from the optimal motion vector set.

To sum up, the motion estimation method according to the presentinvention is particularly designed to find the best matching block orblock pair. By changing the sizes and/or directions of the predeterminedmotion-vector sets to define a plurality of candidate blocks, aplurality of block difference values can be obtained. One of theplurality of block difference values, e.g. the smallest one, isselected, and the motion-vector set corresponding to the selected blockdifference value is then determined as the optimal motion vector set.Since the current field and the preceding and next fields are allreferred to when determining the optimal motion vector set, thede-interlacing process can be performed with improved precision.Furthermore, by using the spatial line-averaging method, instead oftemporally de-interlacing method, to estimate the missing pixels of thepreceding and next fields, error propagation can be avoided.Furthermore, by setting thresholds for the block difference values tomodify the motion estimation method, the de-interlacing effect forspecial cases can be further improved.

While the invention has been described in terms of what is presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the invention needs not to be limited to the aboveembodiments. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

What is claimed is:
 1. A motion estimation method for use in ade-interlacing process of a current interlaced field, comprising:defining a specified block in the current interlaced field, wherein thespecified block contains at least one missing pixel and at least oneexisting pixel; pre-estimating missing pixels of a prior interlacedfield and a subsequent interlaced field; defining a first candidateblock in the prior interlaced field with pre-estimated pixel values anddefining a second candidate block in the subsequent interlaced fieldwith pre-estimated pixel values, wherein a position of the firstcandidate block in the prior interlaced field and a position of thesecond candidate block in the subsequent interlaced field are determinedaccording to a position of the specified block in the current interlacedfield and a motion-vector set; calculating a first block differencevalue, which is a first difference between the at least one existingpixel of the specified block and at least one corresponding pixel of thefirst candidate block with pre-estimated pixel values, wherein the firstdifference is not between any one missing pixel of the specified blockand a corresponding pixel of the first candidate block withpre-estimated pixel values; calculating a second block difference value,which is a second difference between at least one existing pixel of thefirst candidate block and at least one corresponding existing pixel ofthe second candidate block, wherein the second difference is not betweenany one missing pixel of the first candidate block and a correspondingmissing pixel of the second candidate block; and calculating a motionestimation value corresponding to the motion-vector set according to thefirst block difference value and the second block difference value. 2.The motion estimation method according to claim 1 wherein the missingpixel is centered in the specified block.
 3. The motion estimationmethod according to claim 1 wherein the prior interlaced field and thesubsequent interlaced field are of the same parity field which isopposite to a parity field of the current interlaced field.
 4. Themotion estimation method according to claim 1 wherein pixel values ofthe missing pixels of the prior interlaced field and subsequentinterlaced field are pre-estimated by interpolation with pixel values ofadjacent pixels in upper and lower existing lines.
 5. The motionestimation method according to claim 1 wherein the first candidate blockand second candidate block have the same pixel size as the specifiedblock.
 6. The motion estimation method according to claim 1 wherein themotion-vector set comprises a first motion vector pointing from acoordinate of the specified block to a first specified coordinate wherethe first candidate block is defined, and a second motion vector beingin parallel to the first motion vector and pointing from the firstspecified coordinate of the first candidate block to a second specifiedcoordinate where the second candidate block is defined.
 7. The motionestimation method according to claim 6 wherein the motion-vector setfurther comprises a third motion vector opposite to the first motionvector and pointing from the coordinate of the specified block to thesecond specified coordinate of the second candidate block.
 8. The motionestimation method according to claim 7 further comprising calculating athird block difference value, which is a third difference between the atleast one existing pixel of the specified block and the at least onecorresponding pixel of the second candidate block with pre-estimatedpixel values wherein the third difference is not between any one missingpixel of the specified block and a corresponding pixel of the secondcandidate block with pre-estimated pixel values, wherein the motionestimation value corresponding to the motion-vector set is calculated bysumming the first block difference value, the second block differencevalue and the third block difference value.
 9. The motion estimationmethod according to claim 1 wherein the at least one corresponding pixelof the prior or subsequent interlaced field to be compared with the atleast one existing pixel for calculating the first block differencevalue is a missing pixel with a pre-estimated pixel value or an existingpixel in the prior or subsequent interlaced field, depending on themotion-vector set.
 10. The motion estimation method according to claim 1further comprising: providing other motion-vector sets with sizes and/ordirections different from those of the motion-vector set; redefining thefirst candidate block and the second candidate block according to eachof the motion-vector sets; and recalculating the first block differencevalue, the second block difference value and the motion estimation valueaccording to the specified block, the redefined first candidate blockand the redefined second candidate block.
 11. The motion estimationmethod according to claim 10 further comprising: comparing the resultingmotion estimation values to select an optimal motion estimation value;and defining one of the motion-vector sets, which results in the optimalmotion estimation value, as an optimal motion-vector set.
 12. The motionestimation method according to claim 11 wherein the optimal motionestimation value is the smallest one of the resulting motion estimationvalues.
 13. A motion estimation method for use in a de-interlacingprocess, comprising: receiving a first interlaced field, a secondinterlaced field and a third interlaced field in sequence; calculatingand filling estimated pixel values of missing pixels of the first andthird interlaced fields; defining a specified block in the secondinterlaced field which comprises a plurality of existing pixels;selecting a motion-vector set from a group of predeterminedmotion-vector sets; defining a first candidate block in the firstinterlaced field at a position relative to that of the specified blockin the second interlaced field according to the selected motion-vectorset; defining a second candidate block in the third interlaced field ata position relative to that of the specified block in the secondinterlaced field according to the selected motion-vector set; generatinga first block difference value by comparing pixel values ofcorresponding pixels of the first candidate block with pixel values ofthe existing pixels of the specified block; generating a second blockdifference value by comparing pixel values of corresponding pixels ofthe second candidate block with pixel values of the existing pixels ofthe specified block; generating a third block difference value bycomparing pixel values of existing pixels of the first candidate blockwith the pixel values of corresponding existing pixels of secondcandidate block; and determining whether the selected motion-vector setis an optimal motion-vector set according to the first, second and thirdblock difference values.
 14. The motion estimation method according toclaim 13 wherein the first interlaced field and the third interlacedfield are of the same parity field which is opposite to a parity fieldof the second interlaced field.
 15. The motion estimation methodaccording to claim 13 wherein the estimated pixel values of missingpixels of the first and third interlaced fields are calculated byinterpolation with pixel values of adjacent pixels in upper and lowerexisting lines.
 16. The motion estimation method according to claim 13wherein the selected motion-vector set comprises: a first motion vectorpointing from a coordinate of the specified block to a first specifiedcoordinate where the first candidate block is defined; a second motionvector being in parallel to the first motion vector and pointing fromthe first specified coordinate of the first candidate block to a secondspecified coordinate where the second candidate block is defined; and athird motion vector being opposite to the first motion vector andpointing from the first specified coordinate of the first candidateblock to a second specified coordinate where the second candidate blockis defined.
 17. The motion estimation method according to claim 13wherein the selected motion-vector set is determined to be optimal if asum of the first, second and third block difference values is smallerthan any other corresponding sum resulting from another motion-vectorset.
 18. The motion estimation method according to claim 13 wherein thefirst block difference value is obtained by summing absolute values ofpixel-value differences between the existing pixels of the specifiedblock and corresponding pixels of the first candidate block; and thesecond block difference value is obtained by summing absolute values ofpixel-value differences between the existing pixels of the specifiedblock and corresponding pixels of the second candidate block.
 19. Themotion estimation method according to claim 18 wherein the correspondingpixels of the first candidate block and the second candidate block areexisting pixels.
 20. The motion estimation method according to claim 18wherein the corresponding pixels of the first candidate block and thesecond candidate block are missing pixels filled with estimated pixelvalues.
 21. A motion estimation method for use in a de-interlacingprocess, comprising: receiving a first interlaced field, a secondinterlaced field and a third interlaced field of a video signal;defining a specified block in the second interlaced field; defining aplurality of pairs of first and second candidate blocks in the first andthird interlaced fields, respectively, according to the specified blockand a plurality of motion vector sets; calculating a plurality of firstblock difference values between existing pixels of the specified blockand corresponding pixels of the first candidate blocks; calculating aplurality of second block difference values between existing pixels ofthe specified block and corresponding pixels of the second candidateblocks; calculating a plurality of third block difference values betweenexisting pixels of the pairs of first and second candidate blocks;determining a provisionally optimal motion vector according to thefirst, second and third block difference values; comparing one of thefirst block difference values, one of the second block difference valuesand one of the third block difference values, which correspond to theprovisionally optimal motion vector, with a first threshold and a secondthreshold lower than the first threshold; and determining whether theprovisionally optimal motion vector set includes an optimal motionvector or not according to the comparing result.
 22. The motionestimation method according to claim 21 wherein the provisionallyoptimal motion vector set is determined to include the optimal motionvector if a first condition is not complied with that both of the secondand third block difference values are greater than the first thresholdand the first block difference value is smaller than the secondthreshold, and a second condition is not complied with that both of thefirst and third block difference values are greater than the firstthreshold and the second block difference value is smaller than thesecond threshold.
 23. The motion estimation method according to claim 21wherein the provisionally optimal motion vector set is determined to notinclude the optimal motion vector set if either a first condition iscomplied with that both of the second and third block difference valuesare greater than the first threshold and the first block differencevalue is smaller than the second threshold, or a second condition iscomplied with that both of the first and third block difference valuesare greater than the first threshold and the second block differencevalue is smaller than the second threshold.
 24. The motion estimationmethod according to claim 23 wherein the optimal motion vector isdetermined solely according to the first block difference value if thefirst condition is complied with.
 25. The motion estimation methodaccording to claim 24 further comprising: calculating a plurality ofmotion estimation values corresponding to a plurality of preset motionvectors, respectively; comparing the plurality of motion estimationvalues to find the smallest motion estimation value; and determining amotion vector resulting in the smallest motion estimation value as theoptimal motion vector.
 26. The motion estimation method according toclaim 23 wherein the optimal motion vector is determined solelyaccording to the second block difference value if the second conditionis complied with.
 27. The motion estimation method according to claim 26further comprising: calculating a plurality of motion estimation valuescorresponding to a plurality of preset motion vectors, respectively;comparing the plurality of motion estimation values to find the smallestmotion estimation value; and determining a motion vector resulting inthe smallest motion estimation value as the optimal motion vector.